Aristotle's essentialism grows out of his account of substance and definition: to define a thing properly, he argued, is to state its essence, the properties without which it would not be the kind of thing it is, as opposed to its accidental properties, which it can gain or lose while remaining the same kind of thing throughout, a human being's rational capacity being essential on Aristotle's own famous example, while a particular human's height or hair color remains merely accidental. This distinction was meant to make science possible in a principled way: knowledge of a kind of thing, for Aristotle, is knowledge of its essential properties and what follows necessarily from them, not a mere catalogue of whatever happens to be true of instances one has observed, which could shift arbitrarily from case to case with no underlying pattern to explain or predict anything.
The view faces persistent difficulties even on its own terms: borderline cases, a fetus, a corpse, a person with severe dementia, put pressure on any claim to have identified a clean, non-arbitrary boundary marking exactly which properties are essential and which merely accidental, and evolutionary biology's picture of species as gradually changing populations rather than fixed kinds has made essentialism about biological species specifically far harder to defend than it was in Aristotle's own static cosmos.